1. Field of the Invention
The present invention relates generally to the field of oxygen scavenging materials. More particularly, it concerns cured oxygen scavenging compositions and methods and components for producing the same.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf-life of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition, such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking. In the packaging industry, several means for limiting oxygen exposure have already been developed, including modified atmosphere packaging (MAP), vacuum packaging and oxygen barrier film packaging.
Another means for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. Incorporation of a scavenger in the package can provide a uniform scavenging effect throughout the package. In addition, such incorporation can provide a means of intercepting and scavenging oxygen as it is passes through the walls of the package, thereby maintaining the lowest possible oxygen level throughout the package.
While much emphasis is being placed on thermoplastic oxygen scavenging compositions for use in packaging, it would also be desirable to have oxygen scavenging compositions that have a thermoset structure. Such thermoset compositions could permit oxygen scavenging technology to be used in certain contexts (e.g., coatings) in which it would be difficult to use a thermoplastic composition. Furthermore, oxygen scavenging compositions that tend to have inherent barrier properties are also desirable. It would also be beneficial to have oxygen scavenging coatings that can both protect a metal container (e.g., a can) from corrosion, while conferring oxygen scavenging benefits to the oxygen-sensitive contents of the container.
The present invention is directed to cured (e.g., thermoset and thermoplastic) oxygen scavenging compositions, oxygen scavenging polymers that can be reacted with curing agents, and methods of preparing cured oxygen scavenging coatings.
One aspect of the invention is an oxygen scavenging composition that can be either thermoplastic or thermoset, and that comprises at least one oxygen scavenging polymer and at least one curing agent. The oxygen scavenging polymer comprises a polymeric backbone, at least two curing functional groups that are terminal groups and at least one oxygen scavenging cyclic moiety having from 5 to 9 carbon atoms in its ring and at least one carbon-carbon double bond in its ring. It is preferred that the curing functional groups of the oxygen scavenging polymer be independently selected from hydroxyl groups, amino groups, carboxylic groups, sulfonic groups, and epoxy groups. The curing agent comprises at least two functional groups that are capable of reacting with the curing functional groups of the oxygen scavenging polymer.
Examples of curing agents that can be employed include melamine crosslinkers, isocyanate crosslinkers, and epoxy crosslinkers. Different curing functional groups are preferred for use with different curing agents, and this is one factor that is considered in selecting useful combinations of curing agents and oxygen scavenging polymers for oxygen scavenging compositions (both thermoplastic or thermoset) of the present invention. For example, isocyanate and melamine crosslinkers can be used to link or crosslink oxygen scavenging polymers that have pendant or terminal hydroxyl groups (e.g., diols). Furthermore, epoxy and melamine crosslinkers can be used to link or crosslink oxygen scavenging polymers that have terminal or pendant acid groups (e.g., carboxylic groups). Furthermore, isocyanate and epoxy crosslinkers can be used in embodiments of the present invention that are either thermoset or thermoplastic. Melamine crosslinkers can be used in thermoset (e.g., crosslinked) cured oxygen scavenging compositions of the present invention.
The present invention involves linkages (e.g., crosslinks in the case of thermoset compositions) between at least the ends (e.g., termini) of separate oxygen scavenging polymer molecules. Furthermore it is preferred that oxygen scavenging carried out by compositions of the present invention is such that the chemical reaction does not result in the cleavage of the backbone of the crosslinked oxygen scavenging polymer, or in the release of compounds that can have undesirable organoleptic effects. Furthermore, the composition can be in the form of a coating or an adhesive.
Another aspect of the invention is a method of preparing a solvent based oxygen scavenging coating that is thermoset (e.g., crosslinked). A coating solution is prepared comprising at least one oxygen scavenging polymer and at least one curing agent. Both the polymer and curing agent are as described above. The polymer and the agent must have functional groups that are capable of reacting with one another. In certain cases, when neither the curing agent, nor the polymer is a low viscosity liquid, the coating solution also comprises a solvent. The coating solution is applied to a surface and is then cured to produce a thermoset coating. In certain embodiments in which the coating solution further comprises an oxygen scavenging initiator, the method can further comprise initiating oxygen scavenging.
Yet another aspect of the invention is a method of preparing a solventless based oxygen scavenging coating. A blend comprising at least one oxygen scavenging polymer and at least one solid curing agent (e.g., solid melamine or epoxy crosslinkers) is prepared. Furthermore, the oxygen scavenging polymer of the blend comprises at least two curing functional groups that are terminal groups and at least one cyclic organic moiety, as described above. It is preferred that the oxygen scavenging polymer used in the blend have a Tg of about 40xc2x0 C. to about 80xc2x0 C. The solid curing agent of the blend comprises at least two functional groups as described above that are reactive with the curing functional groups of the oxygen scavenging polymer. After its preparation, the blend is ground to a powder. This powder is applied to a surface, and then cured. In certain embodiments, the method can further comprise initiating oxygen scavenging.
Still another aspect of the invention is an oxygen scavenging composition comprised of at least one type of cured oxygen scavenging polymer. The composition can be either thermoplastic or thermoset. The crosslinked oxygen scavenging polymer comprises a polymeric backbone, at least one oxygen scavenging cyclic moiety having from 5 to 9 carbon atoms and at least one carbon-carbon double bond in its ring. The crosslinked polymer is the product of a reaction of at least one low molecular weight oxygen scavenging polymer and at least one curing agent. The curing agent is as described above. The low molecular weight oxygen scavenging polymer comprises a polymeric backbone, at least two curing functional groups that are terminal groups and at least one oxygen scavenging cyclic moiety having from 5 to 9 carbon atoms in its ring and at least one carbon-carbon double bond in its ring, that is the same as the oxygen scavenging cyclic moiety of the crosslinked polymer. The composition can further comprise one or more additional components, as described above. Furthermore, compositions of this embodiment can be either solvent based or solventless based oxygen scavenging compositions.
Another aspect of the invention is an oxygen scavenging polymer, capable of being crosslinked having curing functional groups that are carboxylic groups or hydroxyl groups, and that is the product of a reaction comprising cis-1,2,3,6-tetrahydrophthalic anhydride and at least one diol. The reaction can further comprise at least one polyol (e.g., triol). Addition of small amounts of a triol or other polyol can be used in the reaction to produce a highly branched polyester. This oxygen scavenging polymer comprises a polymeric backbone and at least two curing functional groups that are terminal groups. It has a number average molecular weight between about 500 Mn and about 5000 Mn. Furthermore, the oxygen scavenging polymer comprises at least one oxygen scavenging cyclic moiety derived from cis-1,2,3,6-tetrahydrophthalic anhydride having a particular configuration of formula (IV), described below, wherein h is 1, j is 0, q10, q11, q12, q13, rxe2x80x3, R9, and R10 are H. The oxygen scavenging polymer can further comprise at least one oxygen scavenging cyclic moiety derived from an alcohol that has a formula selected from formula (I), (II), (III), and (IV), described below in more detail. Preferably between about 2 and 16 wt % of the total xe2x80x94CHxe2x95x90CHxe2x80x94 units in the oxygen scavenging polymer are those present in the oxygen scavenging cyclic moieties (e.g., having formula (I), (II), (III), or (IV), while the remainder of the xe2x80x94CHxe2x95x90CHxe2x80x94 units are present in the polymeric backbone of the low molecular weight oxygen scavenging polymer.
Certain of the oxygen scavenging compositions of the present invention are thermoset and are therefore useful in certain contexts in which thermoplastic compositions have limited utility. For example, certain thermoset compositions of the present invention can be used as adhesives to join articles or layers together. They can also be particularly useful as coatings for metal containers (e.g., cans).
Certain ordinary (e.g., non-oxygen scavenging) thermoset compositions can be used to treat the surface of a can or a squeezable tube to protect it from corrosion. Corrosion of a metal food can can cause food contained in it to take on an unpleasant flavor. Metal corrosion can be a particular problem with certain foods that are relatively reactive with metal and that are packaged in metal cans (e.g., carbonated beverages and acidic juices). When certain compositions of the present invention are applied to the inner surfaces of cans, they can help prevent the metal from being corroded, while providing the capacity to scavenge oxygen, thus helping to preserve the oxygen sensitive goods in the container and prolonging their potential shelf life. Certain compositions of the present invention can also be applied to the inner surfaces of squeezable tubes, such as those used for packaging paints, toothpastes, pharmaceuticals and cosmetics. Such squeezable tubes can be made from metal, plastic or paper. Increasing a good""s shelf life can provide cost savings, in reducing the need to restock and in decreasing waste. Certain compositions of the present invention could also be used to in the field of electronics to protect oxygen sensitive electronic components.
A general advantage of thermoset materials is that they tend to inherently possess good barrier properties (e.g., gas barrier and moisture barrier properties), and thermoset compositions of the present invention have enhanced barrier properties, because of their added ability to scavenge oxygen.